1. Field of the Invention
This invention relates to an organic electroluminescent device using a hole-injectable light-emitting material, excellent in light-emitting properties and particularly in low-voltage driving.
2. Description of the Related Art
An organic electroluminescent device (referred to hereinafter as xe2x80x9corganic EL devicexe2x80x9d) is a light-emitting device which makes use of the principle that when an electric filed is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. Since C. W. Tang et al. of Eastman Kodak Company reported a low-voltage-driven organic EL device using a double layered structure (C. W. Tang, S. A. VanSlyke, Applied Physics Letters, 51, 913 (1987) and the like), studies on an organic EL device have been briskly carried out. Tang et al. reported an organic EL device using tris(8-hydroxyquinolinol aluminum) in an electron-transporting and light-emitting layer and a triphenyldiamine derivative in a hole injectable layer. This stacked structure gives such advantages as an improvement in the injection efficiency of holes into the light-emitting layer and blocking of electrons injected from a cathode, which increase the efficiency of exciton production from charge recombination. A double layered structure composed of a hole-injecting and transporting layer and an electron-transporting and light-emitting layer or a triple layered structure composed of a hole-injecting and transporting layer, an electron-transporting and light-emitting layer, and an electron-injecting and transporting layer is well known as an organic EL device. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices.
As a hole injectable material, triphenylamine derivatives such as 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine and aromatic diamine derivatives such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine, which are starburst molecules, are well known (e.g., Japanese Patent Application Laid-Open Nos. 20771/1996, 40995/1996, 40997/1996, 53397/1996 and 87122/1996).
As an electron-transporting material, oxadiazole derivatives, triazole derivatives and the like are well known.
As an electron-transporting and light-emitting material, chelate complexes such as tris(8-quinolinolate)aluminum complex are known, and it is also well-known that devices using the mixture of a light-emitting material and a small amount of a fluorescent pigment have an improved efficiency. As these fluorescent pigments, there are known coumarin derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives, oxadiazole derivatives, porphyrin derivatives, phenoxazoline type pigments, rhodamine type pigments, acridine type pigments and the like. It is reported that various color light in a visible region from blue to red are obtained from these materials, and realization of a full color organic EL device is expected (e.g., Japanese Patent Application Laid-Open Nos. 239655/1996, 138561/1995, 200289/1991 and the like).
However, although the emission efficiency of a triple or more layered organic EL device is high, the driving voltage is increased due to the increase of carrier injection barriers and film thickness as a whole. Further, when the conventional electron-transporting and light-emitting layer is used, there is the disadvantage that the operating voltage is increased due to a high barrier to injection of holes into the light-emitting layer.
In recent days, organic EL devices with high brightness and long lifetime have been disclosed or reported, but these improvements are not enough. There is thus a strong need for development of materials having excellent properties.
The object of the present invention is to provide an organic EL device with high brightness, long lifetime and low operating voltage, as well as a hole-injectable light-emitting material for constituting this device.
As a result of their eager study to solve the problems described above, the present inventors found that aromatic diamine compounds having specific styrylphenyl groups, shown in the general formulae [1] and [2], or [3] and [4], have high charge-transporting properties caused by the styryl group, and have functions as organic electroluminescent materials when a suitable arylene group between two amino groups is selected. The present inventors also found that the styryl group and the styryl group substituted with a diarylamino group of said aromatic diamine compounds permit holes to be easily injected from indium tin oxide generally used as transparent electrodes or metals such as gold and copper into said aromatic diamine compounds, too. Further, the present inventors found that good EL properties are found in devices having a light-emitting zone constituted of a plurality of layers in which the compounds according to the present invention are used in a layer in contact with the anode and a layer composed of the above-described known light-emitting materials that has been inserted between said layer and the cathode. It was accordingly found that organic EL devices with a light-emitting layer made of the light-emitting materials in contact with the anode emit with higher brightness at lower voltage, and also that their lifetime of emission in the case of continuous driving are longer than in the prior art. Furthermore, the present inventors found that said device using a specific electron-transporting material emits at lower voltage and with higher efficiency because of the preferable balance of injection of holes and electrons into the light-emitting layer. In addition, the wavelength of emission light can be selected by selecting arylene groups for Ar1 in the general formula [1] and for Ar6 in the general formula [3], thus enabling selection of various colors of emission light such as bluish violet, green, yellow, white, orange and red, and the present invention was thereby completed.
An organic electroluminescent device according to the present invention comprises at least an anode, an organic light-emitting zone which consists of one or more than one organic thin-film layers and a cathode as elements, wherein said organic light-emitting zone is adjacent to the anode, and a layer contacting the anode contains, either singly or as a mixture, a compound represented by the following formula [1]: 
wherein Ar1 represents a substituted or unsubstituted arylene group having 5-42 carbons, Ar2 to Ar5 each independently represent a substituted or unsubstituted aryl group having 6-20 carbons, at least one of Ar2 to Ar5 is a styrylphenyl group represented by the following general formula [2], whereupon substituent groups thereof are an alkyl group having 1-6 carbons, an alkoxy group having 1-6 carbons, an aryl group having 6-18 carbons, an aralkyl group having 7-8 carbons, an aryloxy group having 6-18 carbons, a halogen atom, a substituted or unsubstituted amino group (excluding a diarylamino group), a hydroxyl group, a cyano group, and a nitro group. 
wherein R1 to R11 each independently represent a hydrogen atom, a halogen atom, a substituted or unsubstituted amino group (excluding a diarylamino group), a hydroxyl group, a cyano group, a nitro group, a alkyl group having 1-6 carbons, an alkoxy group having 1-6 carbons, a substituted or unsubstituted aryl group having 6-18 carbons and a substituted or unsubstituted aryloxy group having 6-18 carbons.
Further, the present invention relates to an organic electroluminescent device wherein a layer contacting with the anode in the light-emitting zone contains, either singly or as a mixture, a compound represented by the following formulae [3] and [4]. 
wherein Ar6 represents a substituted or unsubstituted arylene group having 5-42 carbons, Ar7 to Ar10 each independently represent substituted or unsubstituted aryl group having 6-20 carbons, at least one and at most three of Ar7 to Ar10 are diarylamino-substituted styrylphenyl groups represented by the following general formula [4], whereupon substituent groups thereof are an alkyl group having 1-6 carbons, an alkoxy group having 1-6 carbons, an aryl group having 6-18 carbons, an aralkyl group having 7-8 carbons, an aryloxy group having 6-18 carbons, a halogen atom, a substituted or unsubstituted amino group (excluding a diarylamino group), a hydroxyl group, a cyano group, and a nitro group. 
wherein at least one of R16 to R20 is a diarylamino group represented by xe2x80x94NR23R24, whereupon R23 and R24 each independently represent a substituted or unsubstituted aryl group having 6-18 carbons, and R12 to R22 excluding the said diarylamino group, xe2x80x94NR23R24, each independently represent a hydrogen atom, a halogen atom, a substituted or unsubstituted amino group (excluding a diarylamino group), a hydroxyl group, a cyano group, a nitro group, an alkyl group having 1-6 carbons, an alkoxy group having 1-6 carbons, a substituted or unsubstituted aryl group having 6-18 carbons and a substituted or unsubstituted aryloxy group having 6-18 carbons.
Further, the present invention relates to said organic electroluminescent device comprising an electron-injecting zone between the organic light-emitting zone and the cathode.
The material according to the present invention is used as a material in a light-emitting zone adjacent to an anode to constitute an organic EL device, thereby achieving emission with higher brightness and higher efficiency at low voltage, so the effects of the present invention are outstanding.